Metallothermic production of magnesium

ABSTRACT

MAGNESIUM IS PRODUCED BY THE METALLOTHERMIC REDUCTION OF MAGNESIUM OXIDE IN A REACTION ZONE CONTAINING A MOLTEN SLAG OF 0-25 PERCENT ALUMIA, 30-50 PERCENT SILICA, 5-30 PERCENT MAGNESIUM OXIDE, AND THE BALANCE CALCIUM OXIDE BY MEANS OF A METALLIC REDUCING AGENT CONTAINING 50-100 PERCENT SILICON, 0-40 PERCENT ALUMINUM AND 0-15 PERCENT IRON, AND IN A REACTION-CONDESATION SYSTEM CONTAINING IN ITS VAPOR SPACE INERT GAS AT A PARTIAL PRESSURE OF 0.25-2 ATMOSPHERES.

United States Patent O 3,698,888 METALLOTHERMIC PRODUCTION OF MAGNESIUM Julian Miles Avery, 47 Old Orchard Road, Chestnut Hill, Mass. 02167 No Drawing. Continuation-impart of application Ser. No. 796,214, Feb. 3, 1969, now Patent No. 3,658,509, dated Apr. 25, 1972, which is a continuation-in-part of application Ser. No. 648,856, June 26, 1967, now Patent No. 3,579,326, dated May 18, 1971. This application Apr. 6, 1970, Ser. No. 26,116 The portion of the term of the patent subsequent to Apr. 25, 1989, has been disclaimed Int. Cl. C22b 45/00 U.S. Cl. 75-67 11 Claims ABSTRACT OF THE DISCLOSURE Magnesium is produced by the metallothermic reduction of magnesium oxide in a reaction zone containing a molten slag of 025 percent alumina, 30-50 percent silica, -30 percent magnesium oxide, and the balance calcium oxide, by means of a metallic reducing agent containing 50-100 percent silicon, 0-40 percent aluminum and 0-15 percent iron, and in a reaction-condensation system containing in its vapor space inert gas at a partial pressure of 0.25-2 atmospheres.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 796,214, filed Feb. 3, 1969, Pat. No. 3,658,509, issued on Apr. 25, 1972, which in turn is a continuation-in-part of co-pending application of US. Pat. No. 3,579,326, filed on June 26, 1967 and issued on May 18, 1971.

BACKGROUND OF THE INVENTION In my several other applications, I have described several major improvements to the production of magnesium by the metallothermic reduction of magnesium oxide. I have described and claimed improvements to the slag composition, US. Pat. No. 3,579,326 of May 18, 1971 therefor, and to the metallic alloy composition, ibid., and Ser. No. 26,117, filed Apr. 16, 1970; and I have described and claimed the novel use of an inert gas atmosphere, Ser. No. 796,214, filed Feb. 3, 1969, and Ser. No. 26,118, filed concurrently herewith. In developing these inventions I have found that they are in several ways related. Thus, for example, the use of an inert gas atmosphere in the reaction-condensation system may require that the reaction pressure of the evolving magnesium vapor be raised to maintain production rate, or fully to take advantage of its benefits, but this can be achieved by the employment of a certain slag composition (which promotes the reaction) or of a reducing alloy of increased reactivity. Similarly, the silicon content of the magnesium product, which could be raised by the use of a high-silicon reductant, can be controlled by the use of an inert gas atmosphere.

It is therefore the primary objective of this invention to achieve a major advance in magnesium technology by combining and improving upon several of the teachings and discoveries of my recent inventions in an optimal combination for the production of magnesium by metallothermic reduction of magnesium oxide.

BRIEF DESCRIPTION OF THE INVENTION The present invention resides in a combination of certain reducing alloy and slag compositions with the use of an inert gas atmosphere at relatively high partial pressure.

The reducing alloy is high in silicon and low in nonreactive metals such as iron, as well as relatively low in aluminum. It consists essentially of about 50-100 percent silicon, 0-40 percent aluminum and 0-15 percent iron.

All percentages and ratios herein are by weight.

The molten oxidic slag, which provides the reaction medium in the furnace, is characterized by a high content of magnesium oxide, and by a relatively low content of alumina and silica. The molten slag consists essentially of about 0-25 percent alumina, 5-30 percent magnesium oxide, 30-50 percent silica and the balance calcium oxide.

The inert gas atmosphere present above the molten slag in the vapor space of the reaction-condensation system comprises such inert gases as hydrogen, helium, argon, neon or the equivalent, and may include a combination thereof. Its partial pressure is relatively high, about 0.25-2 atmospheres.

I have found that the combination of this invention provides a highly eflicient process for the metallothermic production of magnesium, having a high yield of magnesium of good purity, with decreased raw material requirements and by-product production, and further that the process is operable at or about atmospheric pressure, and, in any event, a high vacuum system is not required.

DETAILED DESCRIPTION OF THE INVENTION The choice of the proper magnesium ore, reducing alloy and slag composition involves numerous considerations, particularly since the three are interrelated. The choice of ore and reducing alloy affects the slag composition, and a chosen slag composition, which affects the reaction kinetics of the process, can necessitate the use of certain ores and alloys.

For example, there are cogent reasons for the use of silicon metal as the reducing agent, but this choice would necessitate either the absence of alumina from the slag or the inconvenient requirement of charging alumina to the furnace. I therefore prefer to employ an alloy containing aluminum but only enough to provide the alumina needed in a slag of relatively low alumina content. Preferably, then, the slag contains only about 5-15 percent alumina, and the alloy contains about 10-30 percent aluminum. But this preference is not to say that silicon metal cannot be used, only that if it is some alumina should be added, unless none is needed in the slag.

Further, the slag contains relatively low silica, primarily to keep the activity of SiO low, which tends to reverse the primary reaction. But with the use of a high-silicon reductant, and its consequent production of silica by the reaction, it becomes important that the magnesium oxide ore contain little or no silica, as is present for example in serpentine. Also, since the magnesium oxide content of the slag is high, preferably the magnesium oxide ore contains magnesia, e.g., calcined magnesite, in place of part or all of the calcined dolomite (an equimolar combination of MgO and 0210) in order most efiiciently to meet this requirement.

The present slag and alloy combination is desirable, because the preferred combinations and certain of the broader combinations obtain at once a high MgO activity, a low SiO activity (about 0.1 or less), a melting point below 1600 C., a slag:magnesium ratio of less than 3:1, and an alloyzmagnesium ratio of about 0.6: 1. All of these factors support the conclusion that an optimal combination has been approached in the present process.

A reaction-condensation system suitable for the present process comprises in sequence:

(1) Feed bins for raw materials,

(2) Pipes or duets with suitable control mechanisms for feeding raw materials to the furnace,

(3) A furnace (reaction zone) in which the reaction takes place, containing space for molten spent alloy; a molten slag bath in which the reaction occurs; a vapor space above the reaction zone; electrodes designed to cause electric current to flow through the slag bath, thus providing by the Joule effect energy required to promote the reaction; and one or more tap-holes to remove molten slag and spent alloy.

(4) A duct connecting the furnace and the condenser, through which magnesium vapor passes, so designed with respect to effective length and cross section as to permit mass transfer by diffusion without excessive pressure drop. The duct of the present Magnetherm furnace, as show in published literature, happens to be suitable at the stated production capacity,

(5) A condenser (condensation zone) in which magnesium vapor is condensed to molten metal by heat transfer,

(6) A receiving pot connected to the condenser which collects the molten magnesium,

(7) A purge system comprising ducts, control valves, and a vacuum (or comparable) pump which by purging a portion of the inert gas exhausts to the atmosphere gases such as H, and CO, which would otherwise accumulate in the system at a slow rate.

Preferably the system also comprises:

(8) Means to remove inert gas from the condensation zone and to recycle it back to the reaction zone, including if desired means to heat the inert gas. Preferably, such means include means to control the relative flow rate of the inert gas recycled and thus indirectly the relative flow rate of the inert gas from the reaction zone to the condensation zone.

In the process of this invention inert gas can be introduced into the system at any point, but preferably it is introduced into the feed bins or the feed ducts at a slow rate just sufiicient to prevent diffusion of magnesium vapor from the furnace space back into the feed ducts or pipes. The pressure of inert gas on the system is thus controlled at the desired level by suitable devices operating in conjunction with the purge pump or recycle means, if any.

The partial pressure of inert gas in the system is de fined as its pressure at the condenser (conveniently measured in the purge or recycle system). The partial pressure of the magnesium vapor in the condenser is approximately the vapor pressure of magnesium at its melting point (about 7 mm.). The pressure of the magnesium vapor in the furnace space is determined by the pressure drop in the duct, which also affects the pressure differential of the inert gas between the furnace and condenser. The partial pressure of inert gas in the condenser and that of magnesium vapor in the furnace may therefore be quite different, but the total pressure on the system at any point will be at least as great as, and roughly equal to, that of the inert gas in the condenser.

The magnesium oxide ore employed in this process contains a major amount of magnesium oxide, i.e. at least 50 mole percent, and preferably contains at least in part magnesia, that is, an ore at least in part free of calcium oxide and silica. Calcined dolomite may be used, but it is preferred to replace it in part or completely with magnesia.

The reducing agent is high in silicon and relatively low in aluminum and nonreactive metals such as iron and titanium. The reducing agent consists essentially of about 50-100 percent silicon, -40 percent aluminum and 0-15 percent iron. The iron content is as low as practical, but especially when aluminum is present it becomes very diflicult to reduce the iron content much below percent if the alloy is produced by submerged arc smelting. Preferably, the reducing agent consists essentially of about 70- 100 percent silicon, 0-25 percent aluminum and 0-5 percent iron. It is also preferred that the aluminum:silicon ratio of the alloy be about 0.1-0.33:1. A typical preferred alloy would contain about 80 percent silicon, 18 percent aluminum and 2 percent iron.

After the reaction, a spent metallic reducing agent is removed along with the slag, having a composition of about 25-60 percent silicon, 40-75 percent iron and other nonreactive metals, and very little aluminum, if any. Preferably, the spent alloy contains about 40-60 percent silicon, so that it might be sold as a useful ferrosilicon byproduct. Moreover, with a high silicon content in the spent reductant, the primary reaction is favored.

The oxidic slag composition is important because it affects the reaction kinetics of the process, and it should have sufiicient fluidity at about 1600 C. to promote adequate contact of the reactants and evolution of magnesium vapor. Here the slag is characterized by a high MgO and relatively low A1 0 and SiO content. The slag is molten at a temperature between about 1400 and 1700 C., and contains about 0-25 percent alumina, 30-50 percent silica, 5-30 percent magnesium oxide and the balance calcium oxide. Preferably, the slag contains about 0-15 percent alumina, 35-45 percent silica, 10-20 percent magnesium oxide and 20-55 percent calcium oxide. In addition, the SiO activity in the slag should be less than 0.3, preferably less than 0.1 (see R. H. Rein et al., 233 Trans. Met. Socy AIME 415, 423-24 (February 1965) and the total of silica and alumina should be less than 60 percent, preferably about 50 percent. A typical, preferred slag contains about 10 percent alumina, 40 percent silica, 15 percent magnesium oxide and 35 percent calcium oxide. Such slag has a melting point of 1350l400 C., and an SiO activity of 005-01. The composition of the molten slag is best determined by analysis of it after removal from the reaction zone.

In the process of the present invention it is highly desirable to maintain in the reaction zone a temperature of at least about 1400 C. to promote good reaction conditions, but temperatures higher than about 1700 C. are undesirable because they create diflicult engineering and operating problems. It is therefore desirable to employ a slag whose melting point is not higher than about 1600 C. in order that enough superheat may be applied to impart sufficient fluidity to the slag without the necessity of excessively high temperature. Thus, a temperature of about 1400-l700 C. in the reaction zone is preferred, although 1n certain instances higher or lower temperatures are suitable and may be desired.

On the other hand, slags of relatively high viscosity can be used in the present process because there is in the furnace no bed of solid material through which the slag must find its way in order to reach the tap hole for removal from the furnace. Thus the problem of slag viscosity is not as great as it is in most metallurgical processes, but it is still a factor requiring attention.

In general, the composition of the slag is determined in the present process by the ratio of aluminum to silicon fed as the reducing agent; the degree of utilization of silicon as reductant, which for reasons of economy should be as high as feasible; and the relative proportions of magnesium oxide fed as magnesia and as dolomitic lime.

The inert gas atmosphere above the slag, in the vapor space of the reaction-condensation system has a relatively high partial pressure of about 0.25-2 atmospheres.

As employed herein, the term inert gas includes those gaseous materials that are non-reactive with the components of the system under the conditions of operation. Because of the high chemical activity of magnesium at elevated temperature, few gases can be considered inert in the present process. Suitable inert gases include the literally inert gases, such as helium, neon, argon and the like. Another non-reactive gas is hydrogen, which is in certain respects desirable. Hydrogen is cheap and easily available, it provides excellent characteristics for heat transfer in the condenser, and it provides a relatively high specific rate of diffusion.

Preferably, the inert gas in substantially static, and the transfer of magnesium vapor from the reaction zone to the condensation zone is predominately by diffusion. By

substantially static inert gas and the passage of magnesium predominately by diifusion, I mean that the movement of the magnesium vapor is faster than the movement, if any, of the inert gas from the reaction zone to the condenser, or that the magnesium vapor passes through the inert gas, rather than vice versa. Thus, the terms are interrelated and together meet the two conditions. But these conditions are very difiicult of measurement and, in part, somewhat functional. Consequently, I prefer to define the terms in a manner more precise: the molal flow rate of the magnesium vapor to the condenser must be greater than that of the inert gas for the inert gas to be substantially static, and preferably at least twice as great. Since the partial pressure of the inert gas is at least 0.25 atmosphere and substantically static, it follows that the magnesium vapor transfer is predominantly by diffusion through the inert gas.

Preferably, also, means are provided to control the flow rate of the inert gas from the reaction zone to the condensation zone. Such means are readily provided by an inert gas recycle system for removing the inert gas from the condenser and returning it to the furance. Suitable means are described in my copending application, filed concurrently herewith, Ser. No. 26,118, incorporated here by reference, wherein the advantages of employing a substantially static inert gas and means to control its flow are described in some detail.

The remainder of the system may be conventional, i.e. the methods of charging the reducing agent and ore, the furnace and condenser construction, and the removal of slag and spent alloy. The inert gas should be purged periodicaly, or continuously, in order to prevent the building of gases developed in the system, such as hydrogen, nitrogen or carbon monoxide.

I claim:

1. A method of producing magnesium by the metallothermic reduction of magnesium oxide in a reaction zone of a reaction-condensation system, at a temperature at about 1400-1700 C., which comprises charging to the reaction zone an ore, containing magnesium oxide, and a metallic reducing agent, consisting essentially of about 50-100 percent silicon, -40 percent aluminum and 0-15 percent iron; removing from the reaction zone the spent metallic reducing agent and a slag composed of about 0-25 percent alumina, 30-50 percent silica, -30 percent magnesium oxide and 0-65 percent calcium oxide; and providing in the vapor space of the reaction-condensation system inert gas at a partial pressure of about 0.25-2 atmospheres.

2. The method of claim 1, wherein the ore comprises at least in part magnesia.

3. The method of claim 1, wherein the metallic reducing agent consists essentially of about -100 percent silicon, 0-25 percent aluminum and 0-5 percent iron.

4. The method of claim 3, wherein the molten slag comprises about 0-15 percent alumina, 35-45 percent silica, 10-20 percent magnesium oxide and the balance calcium oxide.

5. The method of claim 3, wherein relative flow rate of inert gas from the reaction zone to the condensation zone is controlled.

6. The method of claim 5, wherein inert gas is removed from the condensation zone nad recycled to the reaction zone.

7. The method of claim 1, wherein the molten slag comprises about 0-15 percent alumina, 35-45 percent silica, 10-20 percent magnesium oxide and 20-55 percent calcium oxide.

8. The method of claim 7, wherein the relative flow rate of inert gas from the reaction zone to the condensation zone is controlled.

9. The method of claim 8, wherein inert gas is removed from the condensation zone and recycled to the reaction zone.

10. The method of claim 1, wherein the relative flow rate of inert gas from the reaction zone to the condensation zone is controlled.

11. The method of claim 10, wherein inert gas is removed from the condensation zone and recycled to the reaction zone.

References Cited UNITED STATES PATENTS 3,520,524 7/1970 Stawarz et al -67 UX 3,441,402 4/1969 Magee et a1. 75-10 R 2,497,096 2/ 1950 Parry 75-67 2,255,549 9/1941 Kruh 75-10 A 3,579,326 5/ 1971 Avery 75-67 3,427,152 2/ 1969 Eisenberg et al 75-67 3,475,162 10/1969 Rhodes et al 75-67 3,567,431 3/ 1971 Schmidt 75-67 GEORGE T. OZAKI, Primary Examiner US. Cl. X.R.

75-10 R, 10 P, 10 A 

